Method and apparatus for preventing backfires in metal vapor rectifiers



Aug. 28, 1934. JONAS ET A 1,971,718

METHOD AND APPARATUS FOR PREVENTING BACKFIRES IN METIAL VAPOR RECTIFIERSOriginal Filed May 29, 1926 Patented Aug. 28, 1934 METHOD AND APPARATUSFOR PREVENT ING BACKFIRES IN METAL VAPOR RECTI- Julius Jonas, Baden, andErwin Kern, Wettingen, Switzerland, assignors to, AktiengesellschaftBrown Boveri & Cie., Baden, Switzerland, a

joint-stock company of'Switzerland Original application May 29, 1926,Serial No. 112,665. Divided and this application February 28, 1930,Serial No. 432,033. In Germany June 2, 1925 3 Claims. (01. 175-363) Thisinvention relates to the prevention of certain undesirable, phenomenaknown as back-fires in apparatus for rectifying alternating current ofthe metallic vapor type, of which mercury vapor 6 rectifiers are anexample. The present application is a division of my co-pendingapplication Serial No. 112,665, filed May 29, 1926.

The general object of the invention is the prevention or elimination ofsuch phenomena in a simple and reliable manner.

A particular object is the provision of a method and apparatus for thepurpose stated, and which produces the desired preventive effect at suchtimes when the tendency for the undesirable phenomena to take place isstrongest.

Another object is the provision ofa method and apparatus for the purposestated which does not introduce parts into the interior of the rectifierwhich might themselves be the source of undesirable phenomena.

Other and furtherobjects will be pointed out or indicated hereinafter,or be obvious to one skilled in the art upon an understanding of theinvention.

In the drawing forming a part of this specificationwe show anarrangement of apparatusythe same being presented for the purpose ofillustratingthe invention. This is not to be construed in any fashion ashavingthe effect of limiting the claims short of the true and mostcomprehensive scope of the'invention in the art.

In the drawing,

Fig. 1 is a diagram of apparatus illustrating the invention, V

Fig. 2 is a curve diagram illustrating the variation of the anode andshield potentials using 40 (really internal short-circuits)v which areusually accompanied by a number of other harmful effects. Thus aback-fire which ends in a dead short-circuit may have such a destructiveeffect on the'material: of the rectifier as to necessitate a completeshut-down. When a back-fire takes place, the current flows from theoriginal cathode (or from one of the anodes) to an. anode which hastemporarily assumed'the functions of a cathode,

and it is therefore clearthat a back-fire will usually tend to startduring the time when the potential of the anode concerned is negative tothat of the cathode; This condition is fulfilled during that portion ofthe cycle when the anode isnot carrying a load current.

causes the anode shields to become positive with thesame voltage as themain secondary winding *of the transformer, but is quite independent ofthe latter and galvanically separated therefrom,

As a general rule the potential taken why an insulated anode shieldliessomewhere between 1 fires by subdividing the anode shields or givingthem certain shapes, but without success, the reason being that the merepresence of the shields cannot prevent the current from passing fromcathode to anode. If a tendency to back-fire exists, the increasingglow-discharge current regard to the anodes so that the flow of currenttowards them is assisted.

The object of the invention is therefore a means for preventingback-fires in metal vaporrectifiers 5 having insulated anodes, accordingto which insulated metal structures arranged in the path of theback-fire discharge (these may be the anode shields themselves) aremaintained at a potential negative to their associated anode over theperiod within each A. C. cycle during which the said anode is notcarrying load current. The potential of the metal structures referred toshall be applied in such a way that the anode is always positive to themetal structure or shields during the period of maximum tendency toback-fire.

The anode shield potential may be applied and varied in the desired wayby various means. As shown, for examplain Fig.1 of thedrawing of theherein identified parent application, Serial No. 112,665, now Patent1,845,841, dated February 16,

1932, the shields are all connected to a special auxiliarystar-connected winding of the transformer supplying the rectifier. Suchauxiliary secondary winding may, for example, be wound for and asuitable source of uni-directional potential is connected in circuitbetween the respective neutral points of the main and auxiliary windingsin 100 i such manner that the neutral point of the main winding will bepositive with respect to the neutral point of the auxiliary winding.

Such described arrangement, although providing satisfactory operation,requires a suitable 105 source of low voltage direct current, which isnot always available, and many case will introduce complications, Thenecessity for a source of direct current supply may be entirely avoided,however,by winding the auxiliary secondary to pro- 110 duce a voltagegreater than the voltage produced by the main secondary winding and notconnecting the neutral points of such windings. It appears at first thatthe potential of the auxiliary winding and consequently that of theconnected shields will still be indefinite, but as soon as an arc isstruck between anode and cathode through the shield, the latter willtake up the potential of the anode then carrying load current. Duringthe period, therefore, that an anode is carrying load current, suchanode and associated shield will be at approximately the same potential.The shields of the anodes not carrying current are then at a potentialdifferent from their respective anodes owing to the different voltagesof the main and auxiliary secondary windings of the supply transformer,and the shields are maintained, therefore, at a potential negative withrespect to the potentials of their associated anodes.

Fig. 2 gives the potential curves for this case, where a is the curve ofthe anode voltage and b the curve of the associated shield. Over theperiod t1-t2 the curves are co-incident, owing to the are forming aconducting path. Over the remaining parts, i. e. the anode non-operatingperiod, of the cycle the curves diverge, the shield potential lyingbelow the anode potential. The maximum divergence occurs at time t3 whencurve a is at its maximum negative value and the difference mm is twicethe difierence between the voltages of the main and auxiliary secondarywindings of the supply transformer.

This embodiment of the invention is illustrated in Fig. 1 of the drawingand shows the conductors NW of a three-phase alternating current circuitconnected with the primary winding P1 of transformer T supplying therectifier G. The transformer includes a main secondary winding Q1connected, as shown, to the rectifier anodes m-ae and an auxiliarysecondary winding Q2 connected, as shown, to the anode shields h1hs,through current limiting resistances w1-w6.

- As will be seen from the drawing, the negative and positive conductorsNg of the direct current output circuit start from the neutral point 01of the main secondary winding Q1 and the cathode K respectively, and theneutral points 01 and 02 are not connected. In the example illustrated,the anode shields are insulated from all electrically conductiveelements of the device and, as will be seen, the auxiliary secondarywinding Q2 has more turns than the main secondary winding Q1 so that theshields hlh6 will be supplied with a higher voltage than the associatedanodes a1a6.

Assuming now that an arc passes from anode (1.1 through the shield hi tothe cathode K, the shield hl will then take the same potential as theanode cm. The potentials of the other shields will then depend on themagnitude and the sign of the potential drop existing between theshields. It will be readily seen, therefore, that in the case underconsideration there must be a potential drop between the anode a4 andthe shield hi which is given by the difierence of two potential drops,namely, the potential drop between the terminals 1 and 4 in winding Q2and the potential between terminals 1 and 4 in winding Q1. Since thepotential drop between 1' and 4' is greater than the potential dropbetween 1 and 4 and 4, 4' and negative with regard to 1, 1', it followsthat the shield h! must be negative with regard to or.

It has been assumed hitherto that the arc (load current) leaving theanode forms a conducting path between anode and shield and thusequalizes the potentials of both. With shields of certain shapes orsizes it may happen, however, that the arc does not come into contactwith the shield at all, and its potential will therefore be indefinite.To ensure that both parts have the same potential the shields are givena certain degree of activity by allowing them to work as anodes on to aloading resistance. For this purpose a relatively large resistance isinserted be tween the neutral point 02 of the winding Q2 which suppliesthe shields, and the cathode. A small current then fiows between theshields and the cathode, taking the form of an arc which unites with themain arc from the anodes. In this way the presence of an electricallyconducting path between anode and shield is ensured.

In applying the foregoing method for preventing back-fires the effect ofvery small reverse currents on the shield potential must be taken intoconsideration. It must be assumed that a certain minimum reverse currentis always passing. The effect of this reverse current is to raise theshield voltage and thus tends to counteract the externally impressednegative charge. There are various ways of rendering these reversecurrents inefiective. For example, a smaller control shield can befitted inside the anode shield proper and maintained at a potentialsomewhat lower than the anode. The outer shield then protects thecontrol shield to a certain extent from radiation from the arc and thusprevents its potential being raised by the reverse current. Another waywould be to provide either the inner or the outer shields, or both, withan insulating coating which must also surround the lead to the shieldfrom the point where it enters the rectifier. This insulating coveringwould prevent reverse currents passing through the shields.

Finally, it is advisable to dispense entirely with all leads in theinterior of the rectifier since back-fires are very liable to start fromthe leadin points. This may be done by adopting a known design in whichthe anodes are housed in tubular members of non-metallic insulatingmaterial which extend outside the rectifier. The shield is then placedround the lower part of the tubular member and connected directly to thecorresponding terminal of winding Q2. The resistances w1w6 may then bedispensed with as no current can fiow through the shields.

The anode shields may be replaced by other devices which when given apotential opposing the flow of current towards the anode eliminate thereverse currents. Equivalent devices may take the form of gratings,grids, rings, etc., in short any insulated metal structure placed in thepath of the back-fire discharge.

What we claim is:

1. In an electrical system of the character described, the combinationwith a vapor rectifier comprising anodes and shields associatedrespectively therewith, of means for impressing upon each anode and theassociated shield similar sine waves of voltage, characterized by thefact that at any instant of time the voltages impressed by said means onthe shields are greater in magnitude than the voltages impressed on theassociated anodes, the relation of said anodes to the said shields andthe connections of said means therewith being such that potential isimpressed on said shields from the associated anodes during theoperating period thereof of such sign and magnitude as to render theshields more negative than the associated anodes during thenon-operating periods of the latter.

2. In an electrical system of the character described, the combinationwith a vapor rectifier comprising anodes and shields associatedrespectively therewith, of a transformer winding connected with andoperable to'impress sine waves of voltage on each of saidanodes, and asecond tial thereof is impressed on the associated shields to therebyrender the potential of the respective shields more negative than thepotential of the associated anodes during the non-operating periods ofthe latter.

3. In an electrical system of the character described, the combinationwith a metal vapor rectifier comprising a plurality of anodes andshields associated respectively therewith, of a transformer windingcomprising a plurality of sections connected respectively with and beingoperable to impress voltages of varying sign and magnitude on saidanodes, and a second transformer winding comprising a plurality ofsections connected' respectively with and being operable to impress onsaid shields voltages corresponding in sign atevery instantof time tothe sign of the voltages impressed on the associated anodes, the saidsections of each of said windings having a star point connection and thevoltages impressed on the said shields by the second said winding beingat every instant of time greater in magnitude than the voltagesimpressed on the associated anodes by the first said winding, theposition of the shields with respect to the associated anodes being suchthat during the operating periods of the latter vthe potential thereofis impressed on the shields associated with the nonoperating anodes byway of the said neutral point connection of the said second transformerwinding to thereby render the shields more negative than the associatedanodes during the nonoperating periods of the latter.

JULIUS JONAS. ERWIN KERN.

